Uniaxial hot pressing is a method of forming densified objects by heating and applying uniaxial pressure against a material such as a powder preform or compact. Hot pressing has been used with a variety of powder materials such as metals, ceramics and combinations thereof.
In a hot pressing process, the material to be hot pressed is usually placed into a die and pressure is then applied by a ram or platen which presses against the material directly or indirectly through some intermediate material. Hot pressing applications have generally been limited to forming shapes presenting flat surfaces and uniform cross sections relative to the direction of uniaxial pressure.
Non-uniform cross sections (relative to the direction of uniaxial pressure) react non-uniformly, and often unpredictably, to the applied pressure and heat. If a cross section contains regions of different materials (i.e., a non-uniform cross section), those materials may undergo differing amounts of shrinkage relative to the axial direction. The region that stops shrinking first may assume the entire applied pressure, thus stopping the platen or ram from traveling further and from applying pressure to the other regions. Those other regions may not fully densify because of this lack of applied pressure.
If a surface of the object to be densified is not flat and uniform in thickness (e.g. has a cavity, ridge, step, etc.), hot pressing with a flat ram or platen may cause lateral deformation or distortion of the surface features. Attempts have been made to use rams shaped to correspond to the steps or cavities in the object. These efforts have been largely unable to achieve uniform density in the resulting object without distortion of surface features.